Currently, 90% or more of hydrogen peroxide production is dependent on an anthraquinone process. This process has a large number of reaction steps, including hydrogenation, oxidation, extraction, purification, etc., and thereby impurities are formed due to side-reactions through individual steps, thus requiring removal and separation thereof. Also, the produced hydrogen peroxide has a low concentration and is thus required to be distilled so as to be further concentrated [J. M. Campos-Martin, G. Blanco-Brieva, J. L. G. Fierro, Angew. Chem. Int. Ed., vol. 45, pp. 6962 (2006)].
In order to solve such problems, research into directly preparing hydrogen peroxide from oxygen and hydrogen is ongoing, but the use of the mixture comprising oxygen and hydrogen causes an explosion hazard. In addition, the produced hydrogen peroxide may easily decompose into water and oxygen because of instability thereof, and also the catalyst used is useful in synthesis of water, making it difficult to obtain high selectivity in the course of synthesis of hydrogen peroxide. Therefore, to solve these problems, research into strong acid and halide additives has been conducted. However, the strong acid and halide additives may cause problems of corrosion of the reactor and may dissolve the metal immobilized on a stationary phase, undesirably decreasing activity of the catalyst. Moreover, the hydrogen peroxide preparation process needs separation and purification.
The catalyst used in the direct preparation of hydrogen peroxide mainly includes a precious metal such as gold, platinum, palladium or the like [P. Landon, P. J. Collier, A. J. to Papworth, C. J. Kiely, G. J. Hutchings, Chem. Commun., pp. 2058, (2002); B. E. Solsona, J. K Edwards, P. Landon, A. F. Carley, A. Herzing, C. J. Kiely, G. J. Hutchings, Chem. Mater., vol. 18, pp. 2689 (2006), J. K Edwards, B. Solsona, E. Ntainjua N, A. F. Carley, A. A. Herzing, C. J. Kiely, G. J. Hutchings, Science, vol. 323, pp. 1037 (2009); S. Chinta, J. H. Lunsford, J. Catal., vol. 225, pp. 249 (2004); Y. Han, J. H. Lunsford, J. Catal., vol. 230, pp. 313, (2005); Q. Liu, J. H. Lunsford, J. Catal. vol. 239, pp. 237 (2006)].
Because the catalyst is prepared using an expensive precious metal, there is a need for studies for preparation which enables repeated use of the catalyst. To this end, research into immobilizing precious metal particles on a stationary phase to prevent dissolution upon reaction has been conducted. In addition to the preparation of hydrogen peroxide, methods of preparing catalysts using precious metals, in particular, immobilization methods are being investigated.
Examples of the immobilization methods include physical adsorption, encapsulation, covalent bonding and electrostatic bonding methods, depending on the interactions between the stationary phase and the catalyst.
The physical adsorption method is comparatively easy and uses Van der Waals interaction. Although this process is easy to do, bonding force between the catalyst and the stationary phase is weak and thus dissolution may easily occur [G. Jacobs, F. Ghadiali, A. Pisanu, A. Borgna, W. E. Alvarez, D. E. Resasco, Applied Catalysis A, vol. 188, pp. 79 (1999)].
The encapsulation method encapsulates the active catalyst in a polymer capsule, so that the reactant and the product are separated from each other to induce material transfer resistance, thereby enabling re-use of the catalyst and ensuring high catalyst stability.
The covalent bonding method uses strong covalent bonding between the stationary phase and the functional group on the surface of the catalyst to achieve immobilization, thus ensuring high stability without dissolution of the catalyst, but requires multiple steps. Also, a bond may be formed at the active site of the catalyst, and the catalyst structure may be deformed to due to strong bonding [Y. Yamanoi, T. Yonezawa, N. Shirahata, H. Nishihara, Langmuir, vol. 20, pp. 1054 (2004)].
The electrostatic bonding method uses ionic bonding between a catalyst and a support, and is thus very simple and suitable for a mass production process. Furthermore, sufficient electrostatic bond strength may minimize the dissolution of the catalyst, and may maximally keep up the intrinsic structure of the catalyst thus retaining catalytic activity [S. Kidambi, J. Dai, J. Li, M. L. Bruening, J. Am. Chem. Soc. vol. 126, pp. 2658 (2004)].
As mentioned above, in techniques for directly synthesizing hydrogen peroxide from oxygen and hydrogen, the development of a catalyst which may exhibit high efficiency under reaction conditions that are able to minimize addition of strong acid and halide while immobilizing a precious metal catalyst on a stationary phase to prevent dissolution is required.